Heretofore, as a process for preparing a 1,5-benzothiazepine derivative (I), the method in which a propionic acid derivative represented by the formula (II): ##STR2## wherein (A) one of R.sup.1 and R.sup.2 is a lower alkyl group or a halogen atom, and the other is a hydrogen atom; R.sup.3 is a lower alkyl group or a lower alkoxy group; and R.sup.4 is a hydrogen atom or an ester residue; or
is heated in a solvent (e.g., xylene) to effect an intramolecular ring closing reaction has been known (U.S. Pat. No(s). 4,567,175 and 4,590,188, Japanese Patent Publication (examined) No. 18038/1978).
However, this method involves the problem of requiring a long period of time (e.g., 12 hours in Examples of the above-mentioned Japanese Patent Publication) for the intramolecular ring closing reaction.